SSS Render element and how to comp it

[kpamir]

Hey Guys

I was wondering if anyone has any idea how to comp the SSS render element properly.

At work we are having some issues getting all the layers to comp together and give the same look as the beauty pass.

We are specifically having issues with the SSS pass that Vray pumps out for us.

The Diffuse pass is essentially grey and the SSS has all of the colour info and the SS info.

Our compositor is saying that there should be more to the SSS pass (ie. Back scatter, front scatter etc.) I personally have very little experience compositing and was hoping someone on here could shed some light as to what the problem may be. I know in MR the SSS can easily be separated into Front, Back etc to help with comp but I am not sure how to go about it with Vray.


Do we have to go about it in a hacky way to get the different elements of the SSS shader separated or is what Vray pumps out what we should be using?

Thanks, hopefully some light can be shed on this problem.


Thanks.

KP

[Kel Solaar]

Hi Khaled,

Most of shading operations are additions or multiplications, for instance a very simple Blinn shader could be parametrized like this:

outputColor = diffuseIntensity * diffuseColor + specularIntensity * specularColor

If you want to add SSS to that Blinn, you just "Add" it to the existing components:

outputColor = diffuseIntensity * diffuseColor + specularIntensity * specularColor + sssIntensity * sssColor

There is a subtlety with MR Skin shader in that the shader by default is not using additions but screen operations to blend the components together, while it's giving a soft result the issue with this is that the screen compositing mode tend to clamp the dynamic range of the output, so it's not advised to use it except for dirty quick work.

Now in your case, without seeing the pass it's a bit hard to tell how to composite it, but my guess is just an "Add" ( or Plus or Linear Dodge in Photoshop ) over your existing stuff.

KS

[kpamir]

Thanks for the quick reply Thomas.

I wish I could post the image but it is for work....and that probably wouldn't be a good idea.

I will do some tests tonight at home and try your suggestions.

Thank you sir.

KP

[kpamir]

We seem to have the passes working correctly for the most part.

Would there be a way of separating the SSS pass into individual front scatter and back scatter passes in V-ray?

Our compositing supervisor is asking for that, but I assume we will have to go about it in a hacky fashion.